Conventional methods for making plate glass include the float method, pressing and rolling. These methods have been used successfully in producing soda-lime glasses and other glasses having a relatively low softening temperature.
For glass materials with high softening temperatures, such as fused silica glass, these methods either cannot be used (such as the float method using tin bath), or must be adapted to accommodate the high processing temperatures.
In many applications, high composition and property homogeneity of the glass used is required. For example, synthetic silica glass for use as optical members in precision lithography tools operating in deep and vacuum UV regions, such as at about 248 nm or 193 nm, is required to have a very high purity, very high compositional homogeneity and property homogeneity in terms of concentrations of metal ions and distribution thereof, OH concentrations and distributions thereof, refractive index and variation thereof, transmission and variation thereof, laser damage resistance, birefringence and variation thereof, fictive temperature and variation thereof, and the like.
The conventional glass forming techniques mentioned above, such as the glass plate forming technologies, cannot be easily adapted for use in making high purity fused silica plate meeting the stringent compositional and property requirements of the demanding applications mentioned above.
High purity synthetic silica glass are typically made via flame hydrolysis methods, such as outside vapor deposition (“OVD”), inside vapor deposition (“IVD”), vertical axial deposition (“VAD”), direct-to-glass methods, and the like. The glasses obtained directly from these processes tend to have compositional and property inhomogeneity within the bulk. For example, striae caused by compositional and/or refractive index variations may exist in glass cylinders obtained by OVD, IVD and VAD processes. Glasses made from these processes oftentimes need to be reshaped to a plate or other configuration before further processing into optical elements. It is highly desired that such striae be removed or minimized during such reshaping for demanding applications, at least in the direction of the use axis of the glass. However, conventional thermal reflow of the glass cylinder or pressing does not reduce the striae to a desired level for many demanding applications. Indeed, direct pressing of an OVD, IVD and VAD fused silica cylinder can lead to striae present and observable in the direction of the optical axis of the glass.
As a result, various methods have been proposed in making fused silica glass having a high level of homogeneity at least in the direction of the optical axis. These methods include reshaping processes and/or homogenization processes. However, these currently existing approaches are limited in their ability to accomplish the task.
For example, Berkey and Moore (U.S. Pat. No. 6,689,516 B2) identified a means to fabricate plates from OVD blanks, however the process is limited to thickness up to ˜16 mm. This process requires use of elaborate fixturing to assist in reshaping (stretching) the glass with secondary thermal processing to provide plate straightening. Other approaches employ the use of molds (U.S. Pat. No. 5,443,607); apply force using various fixture designs (U.S. Pat. Nos. 4,358,306, 5,443,607, United States Patent Application Publication Nos. 20030115904A1 and 20030115905A1) to provide means for homogenizing the fused silica glass via reorienting, twisting or mixing striae and/or compositional gradients. The use of these methods either are limited in terms of the maximum mass, relative effectiveness for striae mixing, or are elaborate methods which may induce inclusions and other defects in the glass due to extensive or multiple twisting and kneading operations on the glass surfaces to get the desired mixing action.
Moreover, it has been found that fused silica glass plates produced by the prior art methods tend to have an undesired level of randomness factor in terms of the fast axis directions of the birefringence map.
Therefore, there remains the need for a process for reshaping and/or homogenizing glass materials, particularly those having compositional and/or property variations in the bulk, wherein the impact of such compositional/property variations are reduced or minimized. There is also a need for high purity fused silica glass having a low level of randomness in terms of the fast axis directions in its birefringence map.
The present invention satisfies these needs.